This invention concerns a magnetic brush developing process for electrostatic latent images and, more specifically, it relates to a magnetic brush developing process for electrostatic latent images using highly resistive magnetic toner.
Various processes have been known for developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing or the like. These developing processes can generally be classified into liquid developing processes and dry developing processes.
The dry developing processes include the two-component developing process employing toner composed of a binder resin and a coloring agent such as carbon black dispersed therein and carriers such as glass beads, and the one-component developing process employing only toner without using carriers. This invention concerns the one-component developing process employing only toner.
In the one-component developing process, various methods such as the fur brush method, the touch-down method and the power cloud method have been known, and many proposals have been recently made for development using magnetic brushes formed by the use of magnetic toner composed of binder resins and ferromagnetic materials dispersed therein. One-component developing processes using magnetic toner can generally be classified depending upon the electric resistance of the toners employed, i.e.:
(1) less resistive toner
(2) highly resistive magnetic toner, and
(3) insulative magnetic toner.
The developing process employing the less resistive magnetic toner uses comparatively conductive or semiconductive magnetic toner of an electric resistance up to 10.sup.8 .OMEGA..cm, and performs development by way of an electrostatic induction method, the details of which are described, for instance, in Japanese Patent Laid-Open No. 4532/1974. While the process can perform development by merely contacting or bringing a magnetic brush of less resistive magnetic toner close to the electrostatic latent images, it involves a problem in that the toner images scatter upon electrostatic transfer thereof onto transfer paper by way of corona transfer, bias transfer or the like. Accordingly, image formation has to be carried out, after the formation of toner images on the photosensitive paper, through direct fixing with no transfer step, or by transfer and fixing on a special transfer paper which is resin-processed and highly electrically insulating.
To overcome the problems in transfer performance, a developing process has been attempted using highly resistive magnetic toner of an electrical resistance between 10.sup.9 -10.sup.16 .OMEGA..cm, or insulating magnetic toner of an electrical resistance higher than 10.sup.17 .OMEGA..cm respectively. Although the transfer performance can be improved with an increase in the electric resistance of the toner and corona transfer is made possible without special transfer paper, particularly with respect to the insulative magnetic toner, the process has a defect in that it degrades the developing performance.
In the developing process employing highly resistive magnetic toner or insulative toner respectively, it is impossible to charge the toner by way of the electrostatic induction method as employed for the less resistive magnetic toner, and it is required to charge the toners by some additional means. Such toner charging methods include friction charging between toner particles or between toner particles and an external frictional charging means (for example, see Japanese Patent Laid-Open Nos. 62638/1975, 26046/1976, 22745/1978, 30339/1978 and 106036/1978), corona charging of the toner (for example, see Japanese Patent Laid-Open Nos. 91742/1978 and 68247/1979), or charge injection to the toner by electrodes (for example, see Japanese Patent Laid-Open Nos. 117432/1975 and 51842/1979), and further polarizing development by the dispersion of highly dielectric material in the toners (for example, see Japanese Patent Laid-Open No. 97742/1977). However, the frictional charging method, although capable or providing satisfactory development to some extent at the initial stage, is not satisfactory in that changes with aging are caused in the charging amount, due to the contamination or denaturing such as the surface oxidation of the charging material, or the charging amount is varied by circumstantial changes such as humidity to degrade developing performance.
In the corona charging method, the charging amount is changed by the contamination of the corotron with toner and the toner is deposited on non-image areas as well due to the large charge imparted by the corotron.
The charge injection method using electrodes has defects in that the charging efficiency is poor, making it difficult to obtain the required amount of charge or, if the required amount could be obtained, the amount of injected charge is varied even with minute scratches on the electrodes, which will be reflected as stripes in the picture images. Further, abnormal discharging is caused by electroconductive impurities.
The polarizing method has a defect in that toner capable of attaining satisfactory polarization cannot be obtained with ease and in that electric charge must be applied previously to the toner upon transfer.
As stated above, the conventional developing method using insulating toner, although capable of satisfying transfer performance to some extent, cannot prevent the degradation in the developing performance, particularly, due to aging or circumstantial changes. The developing process using highly resistive magnetic toner, although capable of somewhat stabilized development, also unsatisfactory in that it exhibits poor transfer performance (particularly in highly humid conditions) and results in the scattering of toner images and a reduction of the transfer efficiency. Thus, the conventional developing processes using insulative magnetic toners or highly resistive magnetic toners, respectively, cannot render developing performance and transfer performance compatible with each other.
This invention further concerns a process for transferring reversed magnetic toner images. The one-component developing process conventionally carries out development by applying a desired polarity to the magnetic toner by means of an external means such as charge injection or corona charging and has advantages in that it is capable of performing reversed development with ease by the selection of the polarity. A novel reversed development process, where magnetic toner is deposited on the portion corresponding to the white area on a negative image original, can take place by forming electrostatic latent images on a photosensitive material by the use of the negative-image original, applying electric charges with a polarity identical to that of the latent images to the magnetic toner, and conducting development while keeping the developing bias at a potential approximately equal to the potential at the non-exposed area. However, it has been discovered that, upon corona transfer of the toner images subjected to the reversed development, fogging results,the transfer efficiency is poor, reducing the image density, or the toner images are scattered even by the use of magnetic toners of high electrical resistance. That is, although satisfactory corona transfer can be attained for normally developed toner images obtained by the use of a positive image original, such undesired phenomena may result from the corona transfer of the toner images developed in the reversed manner using the same magnetic toner.